Process For Producing Natural Gas And Natural Gas Condensate From Underground Gas Condensate Deposits

ABSTRACT

A process for producing natural gas and/or natural gas condensate from an underground gas condensate deposit comprising a gas mixture having retrograde condensation characteristics, comprising at least the process steps of:
         a) sinking at least one production well into the underground gas condensate deposit and producing natural gas and/or natural gas condensate from the underground production well through the at least one production well,   b) injecting a solution (S) comprising a solvent and urea through the at least one production well into the underground gas condensate deposit,   c) waiting for a rest phase in which the urea present in the solution (S) is hydrolyzed,   d) producing natural gas and/or natural gas condensate from the underground gas condensate deposit through the at least one production well.

The present invention relates to a process for producing natural gasand/or natural gas condensate from underground gas condensate depositscomprising a gas mixture having retrograde condensation characteristics.

Gas mixtures having retrograde condensation characteristics, coming fromthe gas phase range, undergo partial condensation as the pressure islowered isothermally and move back over to the gas phase as the pressureis lowered further. In general, retrograde condensation characteristicsoccur in a gas mixture whose temperature is above the criticaltemperature of the gas mixture. Natural gas mixtures comprising, forexample, as well as methane, ethane, propanes and butanes, 2 to 20% byvolume of heavy hydrocarbons (C₅+; for example pentanes and hexanes)generally have retrograde condensation characteristics. The phasecharacteristics of gas mixtures having retrograde condensationcharacteristics are shown by way of example in FIG. 1.

In the development of gas condensate deposits comprising gas mixtureshaving retrograde condensation characteristics (also referred to asretrograde gas condensate deposits), the condensation characteristics ofthe above-described retrograde gas mixtures lead to problems. As naturalgas and/or natural gas condensate is withdrawn from such depositsthrough a production well, the pressure in the deposit is reduced, whilethe temperature of the deposit remains very substantially unchanged.This quasi-isothermal lowering of the pressure in the deposit results inpartial condensation of the natural gas present in the deposit. Thelowering of the pressure is at its most marked in the vicinity of theproduction well (near-well zone). As a result of the partialcondensation, especially in the region of the near-well zone, a liquidgas condensate is formed. This liquid gas condensate can block thenear-well zone, greatly reducing the production rate of natural gasand/or natural gas condensate through the production well or evenstopping it completely. This effect is particularly marked in the caseof production of natural gas and/or natural gas condensate depositshaving low permeability.

The blockage of the porous rocks in the region of the near-well zonegreatly restricts the flow of natural gas and/or natural gas condensateto the production well or even stops it completely. Depending on thegeological properties of the deposit and the pressure and temperatureconditions in the deposit, the region in which the liquid gas condensateblocks the flow of natural gas and/or natural gas condensate to theproduction well may be 5 to 100 m in width. The region in which theblockage by the liquid gas condensate is brought about generally has aquasi-cylindrical shape with the production well in the center. Thelowering of the deposit pressure which occurs as a result of theproduction and through the associated blockage with liquid gascondensate can in some cases even lead to the loss of the deposit.

The prior art describes processes which lead to a reduction in theformation of liquid gas condensate and to an improvement in theproduction of natural gas and/or natural gas condensate from a gascondensate deposit.

RU 2018639 describes a process for preventive avoidance of the formationof liquid gas condensate in a gas condensate deposit. The processdescribed therein is also known as a “cycling process”. This involves,in the course of gas production, separating the heavy hydrocarbons (C₅+)above ground from light hydrocarbons (for example methane, ethane andpropanes). The light hydrocarbons are injected back into the deposit as“dry gas”, in order to increase the deposit pressure. The “cycling”process is very inconvenient and costly. In addition, this processcannot reliably avoid the formation of liquid gas condensate in gascondensate deposits.

SU 605429 describes a process for development of gas condensatedeposits. In this process, the deposit is flooded with highlymineralized water. The high mineralization prevents the dissolution ofgases in the flooding water and thus allows the displacement of thenatural gas and of the natural gas condensate from the region of thenear-well zone of the production well. A disadvantage of this process isthe massive watering-out of the deposit as a result of the floodingwater injected. In addition, the flooding water injected can itself leadto blockage of the near-well zone. This process does not enableeffective enhancement of the production rates.

SU 1596081 and RU 2064572 disclose processes in which the gas condensatedeposit is treated with seismic waves. The seismic waves are supposed tolead to an increase in the production rate from the gas condensatedeposit. Especially in the case of low-lying deposits, this process isnot very efficient.

RU 2415257 describes a process for stimulating the production rates ofgas condensate deposits by means of electromagnetic waves. This processtoo is unsuitable, especially for low-lying deposits.

RU 2245997 discloses a process in which solvents are injected at cyclicintervals into the near-well zone, in order to dissolve the liquidcondensate. The solvents used for this purpose are aqueous mixtures ofacetone and methanol, chloroform and methanol or acetone and chloroform.A disadvantage of this process is that the aqueous mixtures introducedlikewise lead to watering-out of the near-well zone. In addition, theprocess is associated with enormous costs due to the organic solventsused. The organic solvents used additionally lead to environmentalproblems resulting from their toxicity.

It was thus an object of the present invention to provide an improvedprocess for producing natural gas and/or natural gas condensate fromunderground gas condensate deposits comprising a gas mixture havingretrograde condensation characteristics. The process shall have thedisadvantages of the prior art described above only to a reduced degree,if at all. The process according to the invention shall be inexpensiveand simple to perform, and lead to an effective increase in theproduction rate of natural gas and/or natural gas condensate from gascondensate deposits after the near-well zone has been at least partlyblocked by liquid gas condensate.

The object is achieved by a process for producing natural gas and/ornatural gas condensate from an underground gas condensate depositcomprising a gas mixture having retrograde condensation characteristics,comprising at least the process steps of

-   -   a) sinking at least one production well into the underground gas        condensate deposit and producing natural gas and/or natural gas        condensate from the underground production well through the at        least one production well,    -   b) injecting a solution (S) comprising a solvent and urea        through the at least one production well into the underground        gas condensate deposit,    -   c) waiting for a rest phase in which the urea present in the        solution (S) is hydrolyzed,    -   d) producing natural gas and/or natural gas condensate from the        underground gas condensate deposit through the at least one        production well.

The object is also achieved by a process for producing natural gasand/or natural gas condensate from an underground gas condensate depositcomprising a gas mixture having retrograde condensation characteristics,comprising at least the process steps of

-   -   a) sinking at least one production well into the underground gas        condensate deposit and producing natural gas and/or natural gas        condensate from the underground gas condensate deposit through        the at least one production well,    -   b) injecting a solution (S) comprising a solvent and urea        through the at least one production well into the underground        gas condensate deposit,    -   c) waiting for a rest phase in which the urea present in the        solution (S) is hydrolyzed,    -   d) producing natural gas and/or natural gas condensate from the        underground gas condensate deposit through the at least one        production well.

The process according to the invention enables the effective enhancementof the production rate of natural gas and/or natural gas condensate froma gas condensate deposit in which the near-well zone has been blocked byliquid natural gas condensate. The process according to the inventionhas the advantage that it works with inexpensive and toxicologicallysafe substances. The process according to the invention preventswatering-out of the near-well zone of the gas condensate deposit.

Process Step a)

In process step a), at least one production well is sunk into theunderground gas condensate deposit. The sinking of the at least oneproduction well into the underground gas condensate deposit is effectedby conventional methods known to those skilled in the art and isdescribed, for example in EP 0 952 300. The production well may be avertical, horizontal or directional well. The production well ispreferably a directional well comprising a quasi-vertical and aquasi-horizontal section.

The gas condensate deposit comprises a gas mixture having retrogradecondensation characteristics. Such gas condensate deposits are alsoreferred to as retrograde gas condensate deposits. The gas mixturepresent in the underground gas condensate deposit comprises generally 80to 98% by volume of light hydrocarbons and 2 to 20% by volume of heavyhydrocarbons. Light hydrocarbons are understood in accordance with theinvention to mean methane, ethane, propanes and butanes. Heavyhydrocarbons are understood in accordance with the invention to meanhydrocarbons having 5 or more carbon atoms, for example pentanes,hexanes and heptanes, and possibly higher hydrocarbons. The terms“propanes”, “butanes”, “pentanes”, “hexanes” and “heptanes” areunderstood in the present context to mean both the unbranchedhydrocarbon compounds and all branched isomers of the above hydrocarboncompounds.

The properties of gas mixtures having retrograde condensationcharacteristics are shown purely by way of example in FIG. 1. The regionlabeled (al) describes the monophasic region in which the gas mixture isexclusively in liquid form. The monophasic region labeled (av) shows theregion in which the gas mixture is exclusively in gaseous form. Theregion labeled (l+v) shows the biphasic region in which one portion ofthe gas mixture is in liquid form and another portion is in gaseousform. (CP) shows the critical point of the gas mixture which connectsthe bubble point curve (bpc) to the dew point curve (dpc).

The bubble point curve (bpc) separates the monophasic liquid region (al)from the biphasic region (l+v). On the bubble point curve (bpc), the gasmixture is virtually 100% liquid and comprises only infinitesimalamounts of gas.

The dew point curve (dpc) separates the monophasic gaseous region (av)from the biphasic region (c+v). On the dew point curve (dpc), the gasmixture is virtually 100% gaseous and comprises only infinitesimalamounts of liquid.

On the horizontal axis is plotted the temperature (T), and on thevertical axis the pressure (P).

A gas mixture having retrograde condensation characteristics undergoespartial condensation as the pressure is lowered isothermally and movesback over to the gas phase as the pressure is lowered further. Theretrograde condensation characteristics generally occur at temperaturesabove the critical point (CP) of the gas mixture. There follows, by wayof example, a description of the behavior of a mixture at a giventemperature above the critical point (CP).

At a given temperature (T₁), the gas mixture having retrogradecondensation characteristics is in gaseous and monophasic form at point(A). As the pressure is lowered isothermally (indicated in FIG. 1 by thedotted line), the gas mixture reaches the dew point curve (dpc) at point(B). At this point, the gas mixture is virtually 100% in gaseous form,but an infinitesimal amount of liquid begins to form. As the pressure islowered further, the gas mixture moves over into the biphasic region(l+v) in which a liquid phase also forms alongside the gas phase as aresult of partial condensation. At point (C), natural gas and liquidnatural gas condensate are thus present alongside one another in abiphasic system. If the pressure is lowered further isothermally, thegas mixture reaches the dew point curve (dpc) again (indicated in FIG. 1by point (D)). Passing over the dew point curve (dpc), the gas mixturemoves back into the monophasic gaseous state. At point (E) in FIG. 1,the gas mixture is again in gaseous and monophasic form. The diagram inFIG. 1 serves merely to illustrate the condensation behavior ofretrograde gas mixtures without restricting the present invention.

The deposit temperature T_(D) of the gas condensate deposits from whichthe process according to the invention produces natural gas and/ornatural gas condensate is typically in the range from 60 to 200° C.,preferably in the range from 70 to 150° C., more preferably in the rangefrom 80 to 140° C. and especially in the range from 85° C. to 120° C.

The deposit temperature T_(D) of the gas condensate deposits must meetthe following conditions:

-   -   1) T_(D) is higher than the crystallization temperature of the        solution    -   2) T_(D) must, within a relatively short period, for example        within 1 to 20 days, allow the full hydrolysis of the urea.

The present invention thus also provides a process in which theunderground gas condensate deposit has a deposit temperature (T_(D)) inthe range from 60 to 200° C., preferably in the range from 70 to 150°C., more preferably in the range from 80 to 140° C. and especially inthe range from 85 to 120° C.

The initial deposit pressure, i.e. the pressure prior to performance ofthe process according to the invention, is typically in the range from80 to 1500 bar; the initial deposit pressure in the case of gascondensate deposits is normally 300 to 600 bar.

The permeability of the underground gas condensate deposits is generallyin the range from 0.01 to 10 mD (millidarcies).

The porosity of the underground gas condensate deposits is generally inthe range from 0.1 to 30%.

After the production well has been sunk into the underground deposit,the deposit pressure is generally at first sufficient to produce naturalgas and/or natural gas condensate through the production well byconventional methods. The terms “natural gas” and “natural gascondensate” in this context do not of course mean a pure hydrocarbonmixture. The natural gas and/or natural gas condensate may of course, aswell as methane, ethane, propanes, butanes, hexanes and heptanes, andpossibly higher hydrocarbons, also comprise other substances.

Further substances may, for example, be sulfur-containing hydrocarbonsor formation water. Formation water in the present context is understoodto mean water originally present in the deposit, and water which hasbeen introduced into the deposit by process steps of secondary andtertiary production, for example what is called flood water. Theformation water also comprises water which may have been introduced intothe gas condensate deposit by the process according to the invention.

A gas mixture having retrograde condensation characteristics has, forexample, the following composition (figures in mol %):

methane 74.6% ethane 8.9% propane 3.8% butane 1.8% pentane 6.4% nitrogen4.5% original density 0.745 g/cm³

“Natural gas” is understood in the present context to mean gaseous gasmixtures which are produced from the gas condensate deposit. “Naturalgas condensate” is understood to mean liquid mixtures which are producedfrom the gas condensate deposit. The state of matter of the mixturesproduced from the gas condensate deposit depends on the temperature andthe pressure in the deposit or in the production well.

By the process according to the invention, it is possible to produceexclusively natural gas through the production well. In addition, it ispossible to produce exclusively natural gas condensate through theproduction well. It is also possible to produce a mixture of natural gasand natural gas condensate through the production well. The state ofmatter of any further substances present in the natural gas or in thenatural gas condensate likewise depends on the pressure and temperaturein the deposit or in the production well. The further substances maylikewise be present in liquid form or in gaseous form in the mixtureproduced through the production well.

If, after the production well has been sunk (process step a)), thedeposit pressure is sufficient to produce natural gas and/or natural gascondensate from the deposit through the production well, this is done byconventional production methods. The present invention thus alsoprovides a process in which, after the at least one production well hasbeen sunk into the underground gas condensate deposit (process step a))and before the solution (S) has been injected into the underground gascondensate deposit (process step b)), natural gas and/or natural gascondensate is first produced (by conventional methods) through the atleast one production well.

However, this is not absolutely necessary. It is also possible toperform process step b) as a preventive measure directly after thesinking of the production well, in order to avoid the formation ofnatural gas condensate.

In general, after process step a), however, natural gas and/or naturalgas condensate is first produced by conventional methods from the gascondensate deposit. As a result of the production of natural gas and/ornatural gas condensate from the gas condensate deposit, the pressure inthe gas condensate deposit decreases, while the temperature of the gascondensate deposit remains very substantially unchanged. Thus, theproduction of natural gas and/or natural gas condensate from the gascondensate deposit leads to an isothermal lowering of the pressure.“Isothermal” is understood in the present context to mean that thetemperature of the gas condensate deposit in the course of performanceof the process according to the invention remains very substantiallyconstant, which means that the temperature of the gas condensate depositchanges by not more than +/−20° C., preferably by +/−10° C. and morepreferably by +/−5° C. in the course of performance of the processaccording to the invention compared to the initial deposit temperatureprior to performance of the process according to the invention.

The lowering of the pressure is at its most marked in the vicinity ofthe production well and decreases with increasing distance from theproduction well. FIG. 2 shows, by way of example, the pressure profilein the underground gas condensate deposit as a function of distance fromthe production well. The distance from the production well is plotted onthe horizontal axis in meters. The deposit pressure (P) is plotted onthe dotted vertical axis. At a particular distance from the productionwell, the deposit pressure (P) reaches a value at which the partialcondensation of the retrograde gas mixture commences. This distance isshown by the vertical dotted line in FIG. 2. At point (B) on the dotteddeposit pressure curve (P), the formation of a biphasic mixturecomprising natural gas and natural gas condensate commences. Point (B)on the dotted deposit pressure curve (P) corresponds to point (B) inFIG. 1. To the left of the dotted line, the gas mixture is in biphasicform ((l+v) region). To the right of the dotted line, the gas mixture isin monophasic form ((av) region).

With onset of the partial condensation, there is a rise in theproportion of liquid natural gas condensate. The proportion of liquidnatural gas condensate is plotted on the vertical axis (CG) and is shownby the solid curve (CG) in FIG. 2. From a certain concentration ofliquid natural gas condensate, the near-well zone is blocked, as aresult of which the production rates of natural gas and/or natural gascondensate from the gas condensate deposit decrease or stop completely.This critical region is shown by the region (CR) shaded gray in FIG. 2.The critical concentration of the liquid natural gas condensate in thegas mixture is shown by the point (CC) on the curve (CG) in FIG. 2. FIG.2 illustrates, merely by way of example, the conditions in a gascondensate deposit comprising a gas mixture having retrogradecondensation characteristics, without restricting the present inventionthereto.

The production of natural gas and/or natural gas condensate from theunderground gas condensate deposit through the at least one productionwell is generally continued until a reduction in the production rate ofnatural gas and/or natural gas condensate is registered.

The reduction in the production rate is attributable to the formation ofthe critical region (CR) at least partly blocked by liquid natural gascondensate.

The present invention thus also provides a process in which theunderground gas condensate deposit prior to performance of process stepb) has a critical region (CR) at least partly blocked by liquid naturalgas condensate.

Prior to the injection of the solution (S) in process step b), theproduction of natural gas and/or natural gas condensate is generallystopped.

The present invention thus also provides a process in which process stepa) comprises the sinking of at least one production well into theunderground gas condensate deposit, the production of natural gas and/ornatural gas condensate from the underground gas condensate deposit untilformation of a critical region (CR) at least partly blocked by liquidnatural gas condensate and the stopping of the production of natural gasand/or natural gas condensate from the underground gas condensatedeposit through the at least one production well.

Process Step b)

In process step b), a solution (S) comprising a solvent and urea isinjected through the production well into the underground gas condensatedeposit.

The solution (S) typically comprises 50 to 79% by weight of urea and 21to 50% by weight of solvent, the solvent comprising water, alcohol or amixture of water and alcohol, based on the total weight of the solution(S). The solution (S) preferably comprises 60 to 78% by weight of ureaand 22 to 40% by weight of solvent, based on the total weight of thesolution (S). The solution (S) more preferably comprises 65 to 77% byweight of urea and 23 to 35% by weight of solvent, based on the totalweight of the solution (S). In a further particularly preferredembodiment, the solution (S) comprises 75 to 77% by weight of urea and23 to 25% by weight of solvent, based on the total weight of thesolution (S).

The present invention thus also provides a process in which the solution(S) comprises 50 to 79% by weight of urea and 21 to 50% by weight ofsolvent, the solvent comprising water, alcohol or a mixture of water andalcohol, based in each case on the total weight of the solution (S).

The solvent used may thus be solely water. It is also possible to usesolely alcohol as the solvent. The use of a solvent comprising onlyalcohol is possible when sufficient formation water is present in theproduction well and/or the underground gas condensate deposit for thehydrolysis of the urea. In addition, it is possible to use a mixture ofwater and alcohol as the solvent. The alcohol used may be exactly onealcohol. It is also possible to use a mixture of two or more alcohols.The alcohol used may be methanol, ethanol, 1-propanol, 2-propanol or amixture of two or more of these alcohols. A preferred alcohol ismethanol.

In a further particularly preferred embodiment, the solution (S)comprises urea and water in a stoichiometric ratio of water to urea of1:1. At this ratio, the urea present in the solution (S) reacts fullywith the water to give ammonia and carbon dioxide. This fully consumesthe water present in the solution (S) and prevents contamination of thegas condensate deposit by water. If the solution (S) comprises solelywater as the solvent, the solution (S) then comprises water and urea ina % by weight ratio of 23.1% by weight of water to 76.9% by weight ofurea.

The solution (S) may consist merely of solvent and urea, withcorresponding application of the above details and preferences. However,it is also possible to add at least one surface-active component(surfactant) to the solution (S). In this case, the solution (S)comprises preferably 0.1 to 5% by weight, more preferably 0.5 to 1% byweight, of at least one surfactant, based on the total weight of thesolution (S).

The surface-active components used may be anionic, cationic and nonionicsurfactants.

Commonly used nonionic surfactants are, for example, ethoxylated mono-,di- and trialkylphenols, ethoxylated fatty alcohols and polyalkyleneoxides. In addition to the unmixed polyalkylene oxides, preferablyC₂-C₄-alkylene oxides and phenyl-substituted C₂-C₄-alkylene oxides,especially polyethylene oxides, polypropylene oxides andpoly(phenylethylene oxides), particularly block copolymers, especiallypolymers having polypropylene oxide and polyethylene oxide blocks orpoly(phenylethylene oxide) and polyethylene oxide blocks, and alsorandom copolymers of these alkylene oxides, are suitable. Such alkyleneoxide block copolymers are known and are commercially available, forexample, under the Tetronic and Pluronic names (BASF).

Typical anionic surfactants are, for example, alkali metal and ammoniumsalts of alkyl sulfates (alkyl radical: C₈-C₁₂), of sulfuric monoestersof ethoxylated alkanols (alkyl radical: C₁₂-C₁₈) and ethoxylatedalkylphenols (alkyl radicals: C₄-C₁₂), and of alkylsulfonic acids (alkylradical: C₁₂-C₁₈).

Suitable cationic surfactants are, for example, the following saltshaving C₆-C₁₈-alkyl, alkylaryl or heterocyclic radicals: primary,secondary, tertiary or quaternary ammonium salts, pyridinium salts,imidazolinium salts, oxazolinium salts, morpholinium salts, propyliumsalts, sulfonium salts and phosphonium salts. Examples includedodecylammonium acetate or the corresponding sulfate, disulfates oracetates of the various 2-(N,N,N-trimethylammonium)ethylparaffin esters,N-cetylpyridinium sulfate and N-laurylpyridinium salts,cetyltrimethylammonium bromide and sodium laurylsulfate.

The use of surface-active components in the solution (S) lowers thesurface tension of the solution (S). This allows the solution (S) tobetter penetrate the regions of the near-well zone blocked by thenatural gas condensate, and to displace the natural gas condensate.

Urea is converted in the presence of water by hydrolysis to ammonia andcarbon dioxide according to the following equation:

H₂N—CO—NH₂+H₂O→2NH₃+CO₂

One mole of urea and one mole of water form two moles of ammonia and onemole of carbon dioxide. The hydrolysis of urea with water under theaction of heat is also referred to as thermohydrolysis. From atemperature of approx. 60° C., the hydrolysis of urea and water proceedswith sufficient rapidity to fully hydrolyze the urea and the water tocarbon dioxide and ammonia within economically viable periods of time.The rate of hydrolysis of the urea present in the solution (S) riseswith increasing temperature.

The solution (S) is typically provided above ground by dissolving theurea in the solvent. It is optionally also possible to add furtheradditives, for example surface-active components (surfactants). The ureais typically used in the form of granules.

In order to accelerate the dissolution of the urea in the solvent andthe preparation of the solution (S), the solution (S) can be heated. Thepresent invention thus also provides a process in which the solution (S)is heated prior to or during the injection in process step b).

The present invention thus also provides a process in which the solution(S) is heated prior to or during the injection in process step b).

The solution (S) can also be used in the form of a true solution (S). Itis also possible to use, as the solution (S), a mixture comprisingsolvent and urea in dissolved form and urea in undissolved form, forexample in the form of crystals. For the process according to theinvention, it is sufficient if the solution (S) can be pumped into thegas condensate deposit by conventional pumps. The solution (S) used ispreferably a true solution.

The dissolution behavior of urea in water is shown in the phase diagramin FIG. 3. The horizontal axis shows the urea content of the solution(S) in % by weight, based on the total weight of the solution (S). Theright-hand vertical axis shows the temperature in ° C. The left-handvertical axis and the dotted curve (1) show the proportion of theresidual water (RW) remaining after the hydrolysis of the urea, based onthe total weight of the solution (S) used.

The dotted vertical line (2) in FIG. 3 indicates the urea concentration(76.9% by weight) at which the water present in the solution (S) isconsumed completely in the hydrolysis of the urea, meaning that theproportion of residual water (RW) remaining after the hydrolysis of theurea is 0. If solutions (S) with relatively low urea concentrations areused, residual water (RW) remains after the hydrolysis of the urea. Theamount of residual water (RW) remaining as a function of the ureaconcentration of the solution (S) used is shown in FIG. 3 by the dottedcurve (1).

The residual water (RW) which remains after the urea hydrolysis if thesolvent used is solely water can be calculated by the following formula:

RW=100% by weight−(KH·1.3)

RW therein states the proportion of residual water (RW) remaining afterthe hydrolysis of the urea in % by weight, based on the total weight ofthe solution (S) used, in the case that solely water is used as thesolvent.

KH therein states the urea content of the solution (S) used in % byweight, based on the total weight of the solution (S) used.

If the solution (S) used is a solution comprising 60% by weight of urea(i.e. KH=60% by weight) and 40% by weight of water (based on the totalweight of the solution (S)), the proportion of the residual water (RW)remaining after the hydrolysis is calculated as

RW=100% by weight−(60% by weight·1.3)=22% by weight

In a preferred embodiment, proceeding from a hypothetical solution (S)comprising only urea and water, the proportion of hypothetical residualwater (RW) in % by weight that would remain in the case of ureahydrolysis in this hypothetical solution (S) is first calculated.

Subsequently, the proportion of hypothetical residual water (RW)calculated in the solution (S) is replaced by the corresponding weightof an alcohol. Suitable alcohols are methanol, ethanol or mixtures ofethanol and methanol, preference being given to methanol. If thesolution (S) comprises less than 76.9% by weight of urea, based on thetotal weight of the solution (S), the solution (S) comprises, as asolvent, preferably a mixture of water and alcohol. The preferred amountof the alcohol corresponds to the proportion of hypothetical residualwater (RW) and is calculated by the following formula:

KA=100% by weight−(KH*1.3).

KA states the preferred amount of the alcohol present in the solution(S).

KH states the urea content of the solution (S) in % by weight.

In the case of urea concentrations of 50% by weight, the solution (S)comprises preferably 50% by weight of urea, 35% by weight of alcohol and15% by weight of water.

In the case of a urea concentration of 55% by weight, the solution (S)comprises preferably 28.5% by weight of alcohol and 16.5% by weight ofwater.

In the case of a urea concentration of 60% by weight, the solution (S)comprises preferably 22% by weight of alcohol and 18% by weight ofwater.

In the case of a urea concentration of 65% by weight, the solution (S)comprises preferably 15.5% by weight of alcohol and 19.5% by weight ofwater.

In the case of a urea concentration of 70% by weight, the solution (S)comprises preferably 9% by weight of alcohol and 21% by weight of water.

In the case of a urea concentration of 75% by weight, the solution (S)comprises preferably 2.5% by weight of alcohol and 22.5% by weight ofwater.

The present invention thus also provides a process in which the solution(S) comprises

-   -   50 to <76.9% by weight of urea,    -   >0 to 35% by weight of alcohol and    -   15 to 50% by weight of water.

The sum of urea, alcohol and water preferably adds up to 100% by weight.

The present invention further provides a process in which the solution(S) comprises

-   -   50 to <76.9% by weight of urea,    -   KA % by weight of alcohol and    -   15 to 50% by weight of water,

where KA is defined by the following formula:

KA=100% by weight−(KH*1.3)

in which KA states the amount of alcohol present in the solution (S) in% by weight and KH states the amount of urea present in the solution (S)in % by weight.

The sum of urea, alcohol and water preferably adds up to 100% by weight.

For the solution (S) used in process step b), the urea concentration ispreferably selected such that the crystallization temperature (T_(C)) ofthe solution (S) is below the deposit temperature (T_(D)) of theunderground gas condensate deposit, the crystallization temperature(T_(C)) being understood to mean the temperature below which ureapresent in dissolved form in the solution (S) crystallizes out, suchthat the solution (S) comprises water, urea in dissolved form and ureain undissolved form.

In other words, the deposit temperature T_(D) is preferably above thecrystallization temperature T_(C) of the solution (S) used. Thecrystallization temperature T_(C) of the solution (S) corresponds, inFIG. 1, to the curve which separates the gray-hatched region “solution”from the region “solution+crystals”. If T_(D) is greater than T_(C), thecrystallization of urea out of the solution (S) in the underground gascondensate deposit can be reliably avoided. The crystallization of ureain the underground gas condensate deposit could lead to blockage of thenear-well zone of the underground gas condensate deposit.

The present invention thus also provides a process in which the solution(S) has a crystallization temperature (T_(C)) below the deposittemperature (T_(D)) of the underground gas condensate deposit.

The present invention further provides a process in which the deposittemperature (T_(D)) of the underground gas condensate deposit is higherthan the crystallization temperature (T_(C)) of the solution (S).

The solutions (S) used are preferably solutions having a ureaconcentration in the range from 50 to 76.9% by weight, based on thetotal weight of the solution (S). At these urea concentrations, 60 to100% by weight of the water originally present in the solution (S) isconsumed in the hydrolysis of the urea. This prevents or at leastreduces contamination of the underground natural gas deposit with water.

The present invention further provides a process in which the durationof the rest phase is selected such that the urea originally present inthe solution (S) is fully hydrolyzed in the underground gas condensatedeposit to carbon dioxide and ammonia, and 60 to 100% by weight of thewater originally present in the solution (S) is consumed.

The present invention thus also provides a process in which the solution(S) comprises 50 to 76.9% by weight of urea and 23.1 to 50% by weight ofwater, based in each case on the total weight of the solution (S).

In a preferred embodiment, a solution (S) comprising 65 to 72% by weightof urea, preferably comprising 69 to 71% by weight of urea, based on thetotal weight of the solution (S), is used. As is evident from FIG. 3,these amounts of urea can be prepared at temperatures in the range from50 to 55° C. to form a true solution (S). The relatively lowtemperatures of 50 to 55° C. have the advantage that the hydrolysis ofthe urea proceeds very slowly at these temperatures, and so nosignificant amounts of ammonia and carbon dioxide are formed. Thesolution (S) is heated by customary heating elements, for example anelectrical heater. The vessels used for production of the solution (S)may, for example, be stirred tanks with a propeller stirrer.

In addition, it is possible first to prepare, at the surface, a solution(S) comprising water, urea in dissolved form and urea in undissolvedform, for example in the form of crystals. This solution cansubsequently be introduced into the gas condensate deposit through theproduction well. In this embodiment, a heating element is present in theproduction well, above the gas condensate deposit, in order to dissolvethe urea present in undissolved form in the solution (S). Such a heatingelement, however, is not absolutely necessary. As stated above, it issufficient if the deposit temperature T_(D) is above the crystallizationtemperature T_(C) of the solution (S). The complete dissolution of theurea in undissolved form is then effected directly in the deposit. Thisembodiment can be selected when the solution (S) is injected intofractures in the underground gas condensate deposit (see, for example,FIG. 4 c, reference numeral 5). Any fracture filled with proppant has avery high permeability and porosity and can “absorb” the crystals.

In a further preferred embodiment, a solution (S) having a urea contentin the range of >72% by weight to 76.9% by weight, preferably in therange from 75% by weight to 76.9% by weight of urea, based in each caseon the total weight of the solution (S), is used. For preparation of atrue solution (S), temperatures in the range from 60 to 70° C. areneeded for this purpose. At these temperatures, noticeable hydrolysis ofthe urea already takes place to form ammonia and carbon dioxide. Inorder to minimize gas formation, it is possible to prepare the solution(S) by briefly heating it to the temperatures needed for dissolution andsubsequently to cool it to temperatures in the range from 50 to 55° C.The brief heating minimizes the formation of carbon dioxide and ammonia.The oversaturated solution (S) thus formed is stable over a prolongedperiod, since the process of crystallization of urea out of theoversaturated solution (S) proceeds slowly. The oversaturated solution(S) can be prepared at the surface as described above and then injectedinto the underground gas condensate deposit through the production well.

In addition, it is also possible to only partly dissolve the urea in thesolution (S) at the surface, such that the solution (S) comprises water,urea in dissolved form and urea in undissolved form. This solution (S)is, as described above, subsequently injected into the underground gascondensate deposit through the production well. In this case, it isagain possible for a heating element to be present in the productionwell above the deposit, such that the urea in undissolved form isdissolved in the solution (S) in the production well. However, this isnot absolutely necessary. It is also possible to inject the solution (S)comprising water, urea in dissolved form and urea in undissolved forminto the underground gas condensate deposit. In this embodiment, theurea in undissolved form is dissolved in the solution (S) in theunderground gas condensate deposit. The above remarks always apply withthe assumption that the deposit temperature T_(D) is higher than thecrystallization temperature T_(C) of the solution (S).

The use of aqueous urea solutions for development of oil depositscomprising viscous oil is described in patent application EP 121 72571,which was yet to be published at the priority date of the presentapplication.

The amount of the solution (S) injected in process step b) depends onthe geological parameters of the underground gas condensate deposit,including the permeability of the deposit and the size of the region(critical region according to FIG. 2) in which the near-well zone isblocked by liquid natural gas condensate. The solution (S) is preferablyinjected in volumes corresponding to not more than the pore volume ofthe critical region (CR) blocked by the liquid natural gas condensate.Suitable volumes of the solution (S) injected in process step b) are inthe range from 1 to 10 m³ per 1 m of the production well surrounded bythe critical region (CR), preferably in the range from 2 to 8 m³, morepreferably in the range from 3 to 7 m³.

The present invention thus also provides a process in which the solutionis injected in process step b) in volumes which lead, in the hydrolysisof urea, to a gas volume of carbon dioxide and ammonia corresponding atleast to the pore volume of the critical region (CR).

In the final phase of the injection of the solution (S) in process stepb), methanol can be added to the solution (S). “Final phase” isunderstood in the present context to mean that at least 90% by weight ofthe solution (S) has been injected, based on the total weight of thesolution (S) injected in process step b).

It is also possible to inject the solution (S) in full and subsequentlyto inject methanol.

This fills the production well with methanol. This facilitates therestarting of the production of natural gas and/or natural gascondensate in process step d).

The present invention thus also provides a process in which, togetherwith the injection of the solution (S) or after the injection of thesolution (S) in process step b), methanol is injected into theproduction well.

The solution (S) described can also be used for flooding of gascondensate deposits. In this case, at least one well is used as acontinuous injection well. The solution (S) is injected into this well.The solution (S) forms gases in the deposit. This process can be usedparticularly efficiently in the development of deposits which have beenabandoned owing to massive dropout of a retrograde gas condensate.

Process Step c)

The injection of the solution (S) is generally followed by a rest phasein which the urea in the underground gas condensate deposit ishydrolyzed to ammonia and carbon dioxide. In a preferred embodiment, theduration of this rest phase is selected such that complete hydrolysis ofthe urea takes place.

The rate with which the hydrolysis of the urea proceeds depends on thedeposit temperature T_(D) of the underground gas condensate deposit andthe temperature with which the solution (S) is injected in process stepb). At high deposit temperatures T_(D), the hydrolysis proceedscorrespondingly more rapidly, and so the rest phase can be selected witha relatively short duration. The duration of the rest phase is generallyin the range from 1 to 10 days. At deposit temperatures T_(D) of 100°C., the rest phase selected may be relatively short, for example 1 to 5days. At deposit temperatures T_(D) in the range from 80 to <100° C.,the duration selected for the rest phase is a range from 5 to 10 days.If the deposit temperature T_(D) is within the range from 60 to <80° C.,the rest phase selected must be correspondingly longer, for example inthe range from 15 to 20 days.

The rest phase results in full hydrolysis of the urea present in thesolution (S) in the underground gas condensate deposit.

During the rest phase, in a preferred embodiment, the production well isclosed. This can be done by customary means, for example packers. As aresult of the closure of the production well, the pressure in thecritical region of the underground gas condensate deposit rises, as aresult of which the efficiency of the process according to the inventionis increased.

The present invention thus also provides a process in which the at leastone production well is closed during the rest phase in step c).

The carbon dioxide formed dissolves partly in the natural gas andpredominantly in the liquid natural gas condensate. This lowers theviscosity of the liquid natural gas condensate, as a result of which themobility of the liquid natural gas condensate in the critical region(CR) of the gas condensate deposit is distinctly enhanced. The ammoniaformed dissolves in the formation water present in the deposit and inthe water injected with the solution (S), and forms an alkaline ammoniabuffer system having a pH of 9 to 10. If the deposit is slightly wateredout, highly alkaline solutions are formed. Under particular conditions,ammonia can also be partly liquefied in the deposit. Liquid ammonia andaqueous ammonia solutions are very good solvents. This additionallyincreases the mobility of the gas condensate.

This buffer system has a surfactant-like effect in the underground gascondensate deposit. This reduces the interfacial tension between thephases, i.e. between the natural gas phase and the liquid natural gascondensate phase and possibly the formation water phase. The formationof the gases (ammonia and carbon dioxide) in the underground gascondensate deposit additionally also has a purely mechanical displacingaction on the liquid natural gas condensate. The lowering of theviscosity of the liquid natural gas condensate and the increasing of themobility of the liquid natural gas condensate facilitate the productionof natural gas and liquid natural gas condensate from the undergroundgas condensate deposit. This distinctly enhances the production rate. Inthe course of production of natural gas, the natural gas also purges theliquid natural gas condensate present in the critical region (CR) of theunderground gas condensate deposit in the direction of the productionwell. This leads to a further enhancement of the production rate.

In a preferred embodiment, in process step b), the solution (S) isintroduced in such amounts that the gas volume formed in the hydrolysisof urea corresponds at least to the pore region of the critical regionof the underground gas condensate deposit.

The present invention thus also provides for the use of a solution (S)comprising water and urea as a means of enhancing the production ratesof natural gas and/or natural gas condensate from a gas condensatedeposit comprising a gas mixture having retrograde condensationcharacteristics. For the use of the solution (S) as a means forenhancing the production rates, the above details and preferences inrelation to the solution (S) apply correspondingly.

Process Step d)

In process step d), natural gas and/or natural gas condensate isproduced from the underground gas condensate deposit, i.e. it isrestarted. The production is effected by conventional methods. Thenatural gas and the natural gas condensate can be produced through theproduction well through which the solution (S) was injected in processstep b) into the underground gas condensate deposit. It is also possibleto sink further wells into the underground gas condensate deposit. Theproduction of natural gas and natural gas condensate can then beeffected through the production well or through the further well. Theproduction well can also fulfill the function of an injection wellthrough which a flooding medium is injected into the underground gascondensate deposit, in which case the actual production is then effectedthrough the one or more further wells. It is also possible to inject aflooding medium through the one or more further wells into theunderground gas condensate deposit and to undertake production throughthe production well through which the solution (S) was injected inprocess step b).

The production of natural gas and/or natural gas condensate from theunderground gas condensate deposit in process step d) is continued untilthe lowering of the pressure which has occurred as a result in theunderground gas condensate deposit leads again to formation of liquidnatural gas condensate, as a result of which the critical region (CR)arises and the production rates decrease significantly. In this case,steps b) and c) are performed again. Steps b) and c) of the processaccording to the invention are thus performed whenever a critical region(CR) which has been blocked by liquid natural gas condensate forms againin the underground gas condensate deposit.

The present invention thus also provides for the use of a solution (S)as a means of enhancing the production rates of natural gas and/ornatural gas condensate from an underground gas condensate depositcomprising a gas mixture having retrograde condensation characteristics.

The present invention is illustrated in detail by the example whichfollows and FIGS. 1, 2, 3 and 4, without being restricted thereto. Themeanings of the reference symbols in the figures are as follows:

-   -   al monophasic liquid region    -   bpc bubble point curve    -   l+v biphasic region    -   dpc dew point curve    -   CP critical point    -   av monophasic gaseous region    -   A, B, C, D and E points in the isothermal lowering of the        pressure of the retrograde gas mixture    -   CG concentration of the liquid natural gas condensate in the gas        mixture    -   CR critical region    -   CC critical concentration of the liquid natural gas condensate        in the gas mixture    -   P pressure    -   T temperature    -   (1) concentration of the residual water after the hydrolysis of        the urea in the solution (S) used    -   (2) concentration of urea at which the water in the solution (S)        is fully consumed in the hydrolysis    -   3 production well    -   4 critical region (CR) blocked with liquid natural gas        condensate    -   5 fracture in the underground gas condensate deposit

The individual figures show:

FIG. 1

The phase behavior of gas mixtures having retrograde condensationcharacteristics.

FIG. 2

The pressure profile and the concentration of liquid natural gascondensate in an underground gas condensate deposit as a function of thedistance from the production well.

FIG. 3

The phase diagram of an aqueous urea solution.

FIGS. 4 a, 4 b, 4 c

Various embodiments of the production well 3.

FIGS. 1, 2 and 3 have already been described in the description of thepresent invention.

FIG. 4 shows different embodiments of a sunk well 3. FIG. 4 a shows avertical production well. The region 4 is the region blocked by liquidnatural gas condensate. FIG. 4 b shows an embodiment in which adirectional well has been sunk. FIG. 4 c shows an embodiment in which adirectional well has been sunk and in which the underground gascondensate deposit has a fracture 5.

EXAMPLE

For development of a gas condensate deposit at a depth in the range from3400 to 3700 m, a directional production well 3 according to FIG. 4 b orFIG. 4 c is sunk. The thickness of the productive stratum is 50 to 80 m.The deposit temperature T_(D) is 105° C. The deposit pressure is approx.650 atm (658.6 bar). The permeability of the deposit is low and isbetween 0.5 and 5.0 mD. After the directional production well 3 has beensunk, it is fracked in the region of the productive stratum, forming afissured zone 5. The porosity of the gas condensate deposit is in therange from 0.2 to 0.25%. The sinking and fracking of the production well3 is followed by commencement of the production of natural gas and/ornatural gas condensate by conventional methods. After a year ofproduction of natural gas and/or natural gas condensate, a significantreduction in the production rate is registered. The reduction in theproduction rate is attributable to blockage of the near-well zone byliquid natural gas condensate. The critical region 4 in which theblockage by liquid natural gas condensate has occurred is estimated tohave a radius of approx. 20 m. The region has a cylindrical shape withthe production well 3 in the center. In order to dissolve the blockage,100 m³ of the solution (S) comprising urea and water, having a ureaconcentration of 70% by weight, based on the total weight of thesolution (S), are injected through the production well 3 into thecritical region 4 of the gas condensate deposit. To prepare the solution(S), 70 t of urea are dissolved in 30 t of water. After the solution (S)has been injected into the gas condensate deposit, the urea ishydrolyzed in the gas condensate deposit, forming approx. 85 000 m³ ofgases (ammonia and carbon dioxide). To prepare the solution (S), urea isused in granule form. To prepare the solution (S), it is heated usingconventional heaters. Directly prior to the injection of the aqueoussolution (S), the solution (S) has a temperature of 50° C., in order toprevent the crystallization of urea out of the solution (S). Thesolution (S) is injected by means of conventional pumps. The injectionof the solution (S) into the gas condensate deposit is followed by arest phase. The rest phase is 3 to 5 days. During the rest phase, theurea is fully hydrolyzed in the underground gas condensate deposit.During the rest phase, the production well is closed. This raises thepressure in the critical region of the underground gas condensatedeposit, increasing the efficiency of the process according to theinvention. In the final phase of the injection, methanol canadditionally be used. This fills the production well with methanolduring the rest phase. This subsequently facilitates the restarting ofproduction. The hydrolysis of the urea results in almost completeconsumption of the water injected into the underground gas condensatedeposit with the solution (S). Blockage of the near-well zone by wateris prevented as a result.

After the rest phase, production is restarted by means of conventionalmethods. The hydrolysis of the urea in the underground gas condensatedeposit distinctly enhances the mobility of the gas mixture present inthe deposit. The natural gas subsequently produced likewise purges anyliquid natural gas condensate still present in the direction of theproduction well. This further reduces blockage of the critical region.After the rest phase, natural gas and liquid natural gas condensate areproduced from the underground gas condensate deposit.

1. A process for producing natural gas and/or natural gas condensatefrom an underground gas condensate deposit comprising a gas mixturehaving retrograde condensation characteristics, comprising at least theprocess steps of a) sinking at least one production well into theunderground gas condensate deposit and producing natural gas and/ornatural gas condensate from the underground production well through theat least one production well, b) injecting a solution (S) comprising asolvent and urea through the at least one production well into theunderground gas condensate deposit, c) waiting for a rest phase in whichthe urea present in the solution (S) is hydrolyzed, d) producing naturalgas and/or natural gas condensate from the underground gas condensatedeposit through the at least one production well.
 2. The processaccording to claim 1, wherein the underground gas condensate deposit hasa deposit temperature (T_(D)) in the range from 60 to 200° C.,preferably in the range from 70 to 150° C., more preferably in the rangefrom 80 to 140° C. and especially in the range from 85 to 120° C.
 3. Theprocess according to claim 1, wherein the solution (S) comprises 50 to79% by weight of urea and 21 to 50% by weight of solvent, the solventcomprising water, alcohol or a mixture of water and alcohol, based ineach case on the total weight of the solution (S).
 4. The processaccording to claim 1, wherein the solution (S) has a crystallizationtemperature (T_(C)) below the deposit temperature (T_(D)) of theunderground gas condensate deposit.
 5. The process according to claim 1,wherein the solution (S) is heated prior to or during the injection inprocess step b).
 6. The process according to claim 1, wherein thedeposit temperature (T_(D)) of the underground gas condensate deposit ishigher than the crystallization temperature (T_(C)) of the solution (S).7. The process according to claim 1, wherein the solution (S) isinjected in process step b) with a temperature higher than thecrystallization temperature (T_(C)), and the deposit temperature (T_(D))of the underground gas condensate deposit is higher than thecrystallization temperature (T_(C)) of the solution (S).
 8. The processaccording to claim 1, wherein the underground gas condensate depositprior to performance of process step b) has a critical region (CR) atleast partly blocked by liquid natural gas condensate.
 9. The processaccording to claim 1, wherein the duration of the rest phase is selectedsuch that the urea originally present in the solution (S) is fullyhydrolyzed in the underground gas condensate deposit to carbon dioxideand ammonia, and 60 to 100% by weight of the water originally present inthe solution (S) is consumed.
 10. The process according to claim 1,wherein the at least one production well is closed during the rest phasein process step c).
 11. The process according to claim 1, wherein thesolution (S) comprises 50 to <76.9% by weight of urea, >0 to 35% byweight of alcohol and 15 to 50% by weight of water.
 12. The processaccording to claim 1, wherein the solution (S) comprises 50 to <76.9% byweight of urea, KA % by weight of alcohol and 15 to 50% by weight ofwater, where KA is defined by the following formula:KA=100% by weight−(KH*1.3) in which KA states the amount of alcoholpresent in the solution (S) in % by weight and KH states the amount ofurea present in the solution (S) in % by weight.
 13. The processaccording to claim 1, wherein the solution (S) is injected in processstep b) in volumes corresponding to not more than the pore volume of thecritical region (CR).
 14. The process according to claim 1, wherein thesolution (S) is injected in process step b) in volumes which lead, inthe hydrolysis of urea, to a gas volume of carbon dioxide and ammoniacorresponding at least to the pore volume of the critical region (CR).15. (canceled)